A commercial field ionization (FI) source has been successfully reconfigured to accommodate the moving belt interface for on-line liquid chromatography-mass spectrometric (LC/MS) operations. This novel combination allows us to characterize saturates with maximum chromatographic resolution and minimal sample preparation. We found that FI generates predominantly molecular ions for normal paraffins and naphthenes but fragment ions for isoparaffins. By combining these ionization characteristics with spectral editing software, we can differentiate and measure levels of naphthenes, normal paraffins, and isoparaffins with a single mass spectrometric (MS) analysis. Most importantly, the normal and isoparaffins split can be measured directly by MS without the need of using gas chromatography (GC). This new LC/MS approach has been evaluated using saturates fractions generated by urea adduction of a lube basestock. Material balance of FI results for the fractions from this separation was in good agreement with that calculated from gravimetric measurement. The normal paraffin measurement by FI agrees well with that by GC. Comparisons with results obtained by the conventional ASTM (American Society for Testing and Materials) high-voltage electron-ionization (EI) method were favorable.